This invention relates to a valve bag filling machine and, more particularly, to an improved spout for a valve bag filling machine.
A machine of the class described is exemplified in U.S. Pat. No. 3,261,379 ('379) which discloses a bin for fluidizing the material to be packaged and a conduit including a filling spout for conducting the material to a bag which is secured to the spout to receive a charge of the material. The bag is weighed and the flow of material is cut off when a predetermined weight is reached by pinching a flexible tube forming a portion of the conduit through which the fluidized material moves from the bin to the filling spout.
Certain materials, such as titanium dioxide and other pigments, tend to build up on the inside of the filling spout as they pass through the spout to the bag. This type of material when in motion is prone to stick to any rigid surface it strikes. As such material passes under pressure through the filling spout it randomly strikes the inner surface of the filling spout and gradually builds up layer upon layer. Most of these pigments have a tendency to build up to a point where they become unstable. In such case material may break off after a certain thickness has accumulated. This can cause bag weight errors due to either or both of the following:
1. The actual weight of a filled bag is influenced by the amount of material that flows into a bag after a balance condition in the scale is reached. This is due to the fact that the material flow cutoff device takes time to react to the scale stop command. The amount of extra material that flows from the time of command to the time all flow stops is directly proportional to the rate of material flow during this time. If buildup is heavy while a bag is being filled, the flow rate is small. So the amount of extra material is small. However, if a chunk of the built-up material breaks away during the beginning portions of the next bag fill cycle, the next bag will have a higher flow rate. So the amount of extra material will be larger. The difference between the weight of the extra amount of material entering the first bag and the weight of the extra material entering the next bag is the bag weight error.
2. By convenience of design, the filling spout that delivers material to a bag is normally mounted on the weighing device. Therefore, any material buildup in the filling spout is sensed by the scale. As a bag being filled approaches its desired weight, all the material on the scale is sensed by the scale. This includes the buildup of material in the filling spout, as well as the material in the bag. However, when the bag is removed from the filling spout and placed on an independent floor scale to check its weight, the material buildup inside the filling spout, which is sensed by the weighing device associated with the filling spout, is not part of the weight sensed by the floor scale. The filled bag weighs less by an amount equal to the buildup in the filling spout. As buildup in the filling spout increases and decreases, that is, breaks away, it causes a proportional increase and decrease in the filled bag weight. This is the bag weight error.
With the new computer weigh systems developed for valve bag filling machines, bag weight accuracy requirements are more stringent. In addition to the bag weight error problems described above, a new problem has surfaced with computer weigh systems. Such a system has the ability to make a weight correction based on the error detected on a previously filled bag. When the computer senses a bag weight error, it makes an adjustment to bring the next bag weight closer to the desired bag weight. It senses the bag weight while the bag is still on the filling spout.
The problem encountered with self-adjusting computer weigh systems is similar to the second problem discussed above in connection with conventional weigh systems. The computer reads the just-filled bag weight while it is still on the scale associated with the filling spout. At this point the computer senses the weight of the material in the bag and the weight of the material buildup in the filling spout. The actual filled weight of the bag, as checked on an independent floor scale, is different from the computer reading by the amount of material buildup in the filling tube. Again, as this buildup varies so does the error between the actual filled bag weight and the computer reading of that bag. So the computer makes adjustments based on erroneous data. This causes even greater variation in bag weight error.
The only practical solution to all the problems described above is to reduce to a negligible amount, or eliminate entirely, the buildup of material in the filling spout.